The invention relates to a display tube comprising a glass envelope having a substantially rectangular display window provided with a phosphor screen, in which window means are present at a short distance from the phosphor screen to cool the screen with a coolant.
Such a display tube is known from German Patent Specification 693,187. During operation of the tube the display window is scanned by an electron beam which causes the phosphor screen to luminesce. However, as a result of the electron bombardment the temperature of the phosphor screen rises so that the luminous efficiency of the phosphor screen decreases. This is the case in particular in display tubes for projection television in which the phosphor screens are scanned by electron beams having large beam currents so as to obtain the required high brightness of the phosphor screens. In order to counteract the decrease of the luminous efficiency it is known to cool the display window and hence the phosphor screen. In the known display tube a meander-like glass cooling tube is provided in the display window at a short distance from the phosphor screen. The display window is manufactured by placing the cooling tube on a thin substrate and then embedding the cooling tube in glass. A gaseous or liquid coolant is passed through the cooling tube.
However, the cooling obtained in this manner is poor, since only a restricted quantity of coolant can be forced through the meander-like cooling tube due to the high flow impedance. Furthermore, for the manufacture of the known display window, various types of glass of different melting-points are necessary which consequently also have different refractive indices. For example, the glass of the cooling tube must have a higher melting-point than the glass in which the cooling tube is embedded. As a result of this, the glass of the cooling tube and the moulded glass will have different refractive indices, so that the cooling tube will be visible. The glass of the thin substrate must also have a higher melting-point than the moulding glass. Moreover, the cooling tube and the glass in which the cooling tube is embedded also have different coefficients of expansion, which may cause stresses in the display window which may lead to fracture of the tube.